- add_filters no longer needs to be called explicitly it will be lazily called if needed.

- the Filter middleware no longer needs to be the first in the handlers array.
- before and after methods will raise an exception if a filter for the requested path is already defined.

src/kemal/middleware/filters.cr

@@ -17,6 +17,11 @@ module Kemal::Middleware
       process_filter(context, :after)
     end
 
+    def filter_for_path_type_defined?(path, type)
+      lookup = @tree.find radix_path(type, path)
+      lookup.found? && lookup.payload.is_a? Block
+    end
+
     private def process_filter(context, type)
       lookup = @tree.find radix_path(type, context.request.path)
       if lookup.found? && lookup.payload.is_a? Block
@@ -28,6 +33,19 @@ module Kemal::Middleware
     private def radix_path(type : Symbol, path)
       "/#{type}#{path}"
     end
+
+    class BeforeFilterAlreadyDefinedException < Exception
+      def initialize(path)
+        super "A before-filter is already defined for path: '#{path}'."
+      end
+    end
+
+    class AfterFilterAlreadyDefinedException < Exception
+      def initialize(path)
+        super "An after-filter is already defined for path: '#{path}'."
+      end
+    end
+
   end
 
   class Block
@@ -35,18 +53,25 @@ module Kemal::Middleware
     def initialize(&@block : HTTP::Server::Context -> _)
     end
   end
+
 end
 
 def add_filters
-  Kemal.config.add_handler Kemal::Middleware::Filter.new
+  unless filter = Kemal.config.handlers.any? { |handler| handler.is_a? Kemal::Middleware::Filter }
+    filter = Kemal::Middleware::Filter.new
+    Kemal.config.add_handler filter
+  end
+  filter
 end
 
 def before(path = "*", options = {} of Symbol => String, &block : HTTP::Server::Context -> _)
-  filter = Kemal.config.handlers.first as Kemal::Middleware::Filter
+  filter = (Kemal.config.handlers.find { |handler| handler.is_a? Kemal::Middleware::Filter } || add_filters) as Kemal::Middleware::Filter
+  raise Kemal::Middleware::Filter::BeforeFilterAlreadyDefinedException.new(path) if filter.filter_for_path_type_defined?(path, :before)
   filter.add :before, path, options, &block
 end
 
 def after(path = "*", options = {} of Symbol => String, &block : HTTP::Server::Context -> _)
-  filter = Kemal.config.handlers.first as Kemal::Middleware::Filter
+  filter = (Kemal.config.handlers.find { |handler| handler.is_a? Kemal::Middleware::Filter } || add_filters) as Kemal::Middleware::Filter
+  raise Kemal::Middleware::Filter::AfterFilterAlreadyDefinedException.new(path) if filter.filter_for_path_type_defined?(path, :after)
   filter.add :after, path, options, &block
 end